1. Field of the Invention
This disclosure relates generally to bandgap voltage reference circuits, and in particular, but not exclusively, those used in the readout circuitry of a complementary metal-oxide-semiconductor (CMOS) image sensor.
2. Background Information
Image sensors are ubiquitous. They are widely used in digital still cameras, digital video cameras, cellular phones, security cameras, medical devices, automobiles, and other applications.
Many image sensors have an image quality that is limited by a number of factors. One such factor is the accuracy of the analog-to-digital converter(s) included in readout circuitry of the image sensor. An accurate analog-to-digital converter often requires a reference voltage that is both accurate and stable. Various circuits can be implemented to provide the reference voltage. One such example is a bandgap reference circuit.
Bandgap reference circuits are widely used in analog circuits for providing stable, voltage-independent, and temperature-independent reference voltages. Bandgap voltage reference circuits operate on the principle of maintaining an internal voltage source that has a positive temperature coefficient and another internal voltage source that has a negative temperature coefficient. By summing the two together, the temperature dependence can be cancelled. Additionally, either of the two internal sources can be used as a temperature sensor.
As the name suggests, the voltages generated by bandgap reference circuits are used as references, and in particular are utilized where high stability in the reference voltage is needed (e.g., as is often required in image sensors). In typical bandgap reference circuits, operational amplifiers are used in order to improve the accuracy of the reference voltages. However, operational amplifiers themselves are not ideal, and have offset voltages. Ideally, voltages at the inputs of the operational amplifier should equal each other due to the virtual short between the inputs of amplifier. However, in practical cases, the offset voltage Vos is almost always present. Furthermore, the offset voltages Vos vary from chip to chip instead of being a fixed value. Thus, the output voltages Vout also vary from chip to chip attributed to the distribution of offset voltages Vos, making it difficult to compensate for such a variation.
Some conventional techniques for compensating for the offset voltages present in operational amplifiers include “high-frequency (HF) chopper techniques,” and “auto-zeroing techniques.” However, such chopper stabilization and auto-zeroing are based on high-frequency switching during the critical row-time of the image sensor. The high-frequency switching may couple high-frequency currents into the bandgap reference output or generate high-frequency voltages in common “ground paths.” Thus, the accuracy of the bandgap voltage output may be reduced which may cause the bandgap reference circuit to be incompatible with image sensors that have high resolution analog-to-digital converters.